FI69004B - ANORDINATION FOER FRAMSTAELLNING AV PLASTROER - Google Patents

Description

69004

Apparatus for making plastic pipes

Anordning förställning av plaströr

The invention relates to a piston injection molding machine for producing a plastic pipe, in which the injection molding machine has a tool heated by heating elements, in this tool a reciprocating piston And a spindle, which determines the inner diameter of the pipe and is fixedly connected to the piston.

High molecular weight low pressure polyethylene (UHMV-PE), with a molecular weight of more than 500,000, differs fundamentally from conventional polyethylenes in its melting and viscosity properties. Polyethylenes having a molecular weight of up to 200,000 change on heating to a melt, which has good flow properties, while UHMW-PE changes as the temperature rises to a viscous-elastic state.

Due to its high viscosity and poor flow properties, this material cannot be formed into pipes in single, twin-screw presses using conventional pipe tools without compromising the properties of the material. Because the mechanical properties of UHMW-PE (strength, stress resistance, high notch impact strength, sliding properties) provide ideal conditions for the production of pipes used in compressed air and hydraulic solids transport (quartz, sand, coal slurry), it has been necessary to develop This allows this material to be transformed into a pipe, so that a new field of application opens up for this material.

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It is already known that continuous plates or rods can be produced from high molecular weight low-pressure polyethylene by means of a piston injection molding machine (see, for example, DE-OS 17 29 174 and DE-PS 17 78 258). On the other hand, it has not been possible to make UHMW-PE pipes with the required dimensional stability in accordance with DIN 8074 and the typical strength properties of UHMW-PE.

Steininger and Stamprech have described the production of polytetrafluoroethylene tubes by piston injection molding in "Extrusionsverarbeitung von Polytetrafluoroethylene-Ftillstoff-Mischungen", Kunststoffe 1975 »page Since the pressures generated by piston injection molding of UHMW-PE are considerably higher than those obtained by piston injection molding of polytetrafluoroethylene, deterioration of the material properties can be expected in the case of UHMW-PE being extruded into pipes according to this known method. The structure of the syringe press itself would become economically unprofitable due to the high operating pressure.

The object of the invention is to manufacture pipes from UHMW-PE in such a way that the typical properties of this material do not change. The dimensions of the pipe must comply with the dimensional tolerances given in DIN 8704. To this end, a device must be developed for piston-vibration compression of plastic pipes in such a way as to avoid the formation of high compression pressures. The structure of the piston press should be as simple as possible and inexpensive.

The invention relates to a piston injection molding machine for producing a plastic tube, the injection molding machine having a tool heated by heating elements, in this tool a reciprocating piston and a spindle which determines the inner diameter of the tube and is fixedly connected to the piston.

The injection press according to the invention is characterized in that a guide sleeve filling the cross-section of the tool is placed on the spindle, which is extruded during the start-up phase together with the formed press to ensure that the cross-section of the press remains the same along the entire length of the tool.

It has been shown that both the transport of plastic powder and its injection molding by solidification into the desired tube

II

3 69004 the combined forward and rearward movement of the interconnected spindle and the piston can be used to perform disturbances in connection with the injection molding system. Thus, it is not necessary that the mandrel be continuously fixed, nor is it necessary for the piston to be maintained in the injection molding position when the mandrel is retracted to its original position, as in Steininger and Stamprech, supra, on page 339 in the right-hand column. in the case of an injection molding machine for polytetrafluoroethylene.

It has been found that when injection molding pipes from UHMV-PE in the injection molding according to the invention, considerably lower compression pressures are generated than when the corresponding size profile is obtained by piston injection molding. In addition to the lower compression pressure generated, the fixed connection between the mandrel and the piston achieves a considerable simplification of the operation of the injection molding machine, since the piston and the mandrel can be operated by means of a single hydraulic device.

With the piston injection molding structure according to the invention, both UHMW-PE pipes and, more generally, piston injection molding plastic pipes can be produced, i.e. all plastics are used which, due to their special material properties, are piston injection molding and thus cannot normally be formed in a helical rui.

Due to the relatively low pressures generated by injection molding with the new piston injection molding machine, this injection molding machine is particularly well suited for the production of pipes from UHMW-P £.

The length of the spindle is usually equal to the length of the tool from the inlet end to the outlet end. The diameter of the mandrel corresponds to the inside diameter of the pipe to be injection molded, taking into account the amount of shrinkage. The spindle, which is fixedly connected to the piston at one end, is not mounted on the outlet side of the tool, as this would adversely affect the formation of a flat tube. For this reason, a guide sleeve surrounding the mandrel is inserted into the tool during the start-up phase, which together with the press is removed from the tool. Thanks to this starting aid, the spindle remains centrally guided until it is completely surrounded by the compaction mass to ensure a uniform wall thickness. The achieved wall thicknesses fall within the tolerance range of the thick-walled 69004 pipes according to the BIN 8074 standard.

In order to keep the roundness of the injection-molded pipe as small as possible, it has proved expedient to Cool the shrinkable pipes as they cool in a controlled manner by the distance immediately associated with the pressing step by which the pipe is guided (calibration). The length of this calibration distance is usually 3 ·· .8 times the outer diameter of the finished pipe,

And the calibration device is either cylindrical or made conically tapered (tr1: 50). Cooling of the calibration plate is conveniently accomplished by cooling the jacket with water at a temperature of 50 to 100 ° C.

The pipes according to the invention can in particular be used for lining steel pipes. To do this, the UHMV-FE pipe is inserted into the steel pipe and its protruding ends are flanged. · This new type of lining of steel pipes is much simpler than conventional pipe lining with UHMW-FE plates, as explained, for example, in DE-AS 24 19 893. Seamless lining prevents penetration of transported goods between the steel pipe and the UHMV-PE liner. This is especially important when transporting media that can adversely affect the steel pipe.

When using pipes lined in this way, the high mechanical strength of the steel and the high wear strength of UHMW-EE as well as the low thermal conductivity can be optimally combined.

Figure 1 of the accompanying drawing shows a piston syringe press according to the invention, consisting of a tool 1 surrounded by heating elements 2, and supported by a support plate 5. Syringe-pressable plastic powder is fed from a hopper 6. . A piston 7 is fixedly connected to the piston 3, which determines the inner diameter of the tube to be injected. The guide sleeve 8 acts as a spindle bearing when the piston vibratory press starts.

Figure 2 shows a steel pipe 9 lined with a pipe 8 made of UHMV-FE. The UHMV-EE pipe is flanged at 11 points on the flanges 10 of the steel pipe 9.

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Example 1

The described method was carried out with a tool having a length of 937 an and an inner diameter of 83 mm. The tool was heated with two electric heating strips, each 350 mm long, with a total power of 3 kV, the temperature being controlled by two temperature controllers. The selected temperature profile of the first end of the heating zone is 220 ° C and the outlet zone enclosed lämmitysvyö-160 ° C. When applying the method without a calibration device, the maximum production pressure of 152 bar was reached with a production volume of 3 * 9 kg UHMV-PE / h injection press. When using the calibration device, the pressure at the same production rate was 280 bar. The dimensions of the injection-molded tube were 100 x 10 mm. The physical properties of UHMW-PE did not change in the injection molding machine.

Example 2

Otherwise, under the same operating conditions as in Example 1, a mandrel having a diameter of 95 mm was used. Using the method without calibration, a maximum pressure of 223 bar was reached in the injection press at a production rate of 7.3 kg UHMW-FE / h. When using calibration, the pressure at the same production volume was 371 bar.